1. Field of the Invention
The present invention relates to a radioactive decontamination method for removing radioactive contaminants from various portions of a nuclear power station or other nuclear facility, such as devices connected to or disposed around a nuclear reactor, pipes and instruments around the nuclear reactor.
2. Description of the Related Art
Radioactive contaminated parts are produced as a result of running of a nuclear facility such as a nuclear power station. More specifically, such radioactive-contaminated parts are produced in the course of replacement or overhaul of various devices and apparatuses in the nuclear power station. These parts are usually packaged in a drum as they are or after cutting into pieces, and are stored in a suitable place such as an abandoned mine.
The amount of stored contaminated parts increases year by year so that storage places such as waste mines will be fully occupied before long. In order to overcome this problem, there has been attempted to build a storage facility at a location which is located far from living areas. Such attempt, however, has encountered opposition of the local residents, thus causing a social problem.
To avert from this problem, varios inventions have been achieved in the field of radioactive decontamination. These efforts are concentrated to reduce the amounts of contaminated parts to be stored, by removing radioactive contaminants from such parts.
One of such inventions is disclosed in Japanese Patent Publication No. 59-36240. According to the invention disclosed in the above-mentioned patent publication, a decontamination medium such as hydrocarbon fluoride or perchloroethylene is heated in a vessel and is vibrated by a supersonic vibrator, and radioactive-contaminated parts are immersed in the decontamination medium, thereby removing radioactive contaminants from the parts.
This known method, however, has the following disadvantage. Firstly, it is to be understood that both hydrocarbon fluoride and perchloroethylene have a small decontamination ability. In fact, these substances exhibit a much smaller cleaning effect comparing these with the present invention, as will be realized from a comparison test result which will be mentioned later.
Secondly, it is to be understood that both hydrocarbon fluoride and perchloroethylene cause environmental pollution.
As is well known, hydrocarbon fluoride breaks down the ozone layer around the earth so as to form ozone holes. The number of persons suffering from cancers is increasing as a result of ultraviolet rays in the solar light rays which reach the earth through the ozone holes without being absorbed by the ozone layer. For these reasons, nowadays, there is a world-wide movement toward prohibition of use of hydrocarbon fluoride. According to the Montreal agreement by members of the United Nations, the production and use of this substance is to be ceased by the end of this century.
Perchloroethylene also has an effect in causing cancer. This substance is toxic when taken into the human body aurally or through contact with skin. Thus, this substance causes a serious pollution. The allowable maximum concentration of perchloroethylene in the air is 100 ppm. In the U.S.A., production and use of perchloroethylene are planned to be ceased by 1996.
In the removal of radioactive contaminants using perchloroethylene, it is necessary to remove, by distillation, contaminants from the decontamination medium, i.e., from perchloroethylene itself. The removal of radioactive contaminants through distillation consumes large electrical power because of the high boiling point (121.2.degree. C.) of the perchloroethylene.
Usually, suitable organic substances are added as stabilizers to perchloroethylene. The content of such stabilizers, however, are reduced in the course of distillation for the cleaning of perchloroethylene. The use of perchloroethylene having such reduced contents of stabilizers adversely affects metals.
Furthermore, perchloroethylene generally exhibits inferior stability to electromagnetic waves. Perchloroethylene is oxidized by ultraviolet rays so as to be changed into trichloroacethyl chloride. When exposed to solar light for a long time, perchloroethylene is easily changed into trichloroacetate and hydrochloric acid.
Thus, perchloroethylene has a risk to be denaturated as a result of application of .alpha.-, .beta.- and .gamma.-rays radiated from radioactive-contaminated parts.